Changes in physical properties of various coir dust and perlite mixes and their capacitance sensor volumetric water content calibrations

Recently developed capacitance soil moisture sensors are suitable for measuring volumetric water content (VWC, v/v) of soilless substrates in real time, useful for substrate moisture based-automated irrigation systems of horticultural crops. For capacitance sensors, a substrate-specific calibration is required for accurate VWC measurements, but sensor manufacturers typically provide only a general calibration for potting soil. However, the general calibration for potting soil is not indicative of the variety of soilless substrates. We investigated and compared the physical properties of various coir dust and perlitemix ratios (1:0, 8:2, 6:4, 4:6, 2:8, 0:1, v/v), and conducted individual substrate-specific VWC calibrations in the range from 0.1 m³·m^-3 to near container capacity. Sensor outputs [in mV and analog-to-digital converter (ADC)] of various VWC levels of the specific substrates were collected with four EC-5 and GS3 sensors (Decagon Devices Inc., Pullman, WA) connected to a data logger with 2.5-V and 12-V excitation, respectively. Substrates with greater relative perlite contents had larger particles, and both the water-holding capacity and electrical conductivity (EC) were lower than the substrate mixes with more coir dust. All tested substrate mixes had linear relationship between EC-5 sensor output and VWC with high R² (>0.93) except in the case of 100% perlite, and their slopes were significantly different across the substrate mixes (P = 0.03). However, most of the substrate mixes had similar calibrations with estimated VWC differences within 0.06 m³·m^-3. TheGS3 sensor had the best fit with a logarithmic relationship between sensor output (in ADC) and VWC, and the slopes among the substrate mixes were not significantly different (P = 0.13). Furthermore, the soilless substrate calibration provided by the company differed from the substrate-specific calibration, resulting in considerable error (up to 0.18 m³·m^-3 difference), during the measurement of the actual VWCs. Therefore, sensor- and substrate-specific calibration is required for accurate VWC measurements with capacitance sensors.